The state of the art covers the need to generate axial thrusts of compression on surfaces subject to movements of displacement on orthogonal planes, or at least planes which do not contain the axis on which the compression thrust is generated.
These displacement movements of the surface subject to compression cause friction between the end of the pressing element and the moved surface, so that anti-friction materials need to be used which, in time, deteriorate and become worn.
Moreover, the chocks of the hydraulic cylinders which drive the pressure elements are subjected to very strong forces, with a consequent premature wear, damage and loss of airtight grip.
There is also the need to ensure that the pressure element is axially centered and correctly repositioned with respect to the area where the compression thrust is applied on the surface subjected to oscillation movements when this oscillation action stops and the surface returns to its original position.
It is also necessary to ensure extensive connection surfaces even when the surface subjected to compression is in a position where it is displaced at an angle, so that the efficiency of the compression action is maintained in all the reciprocally angled positions.
A typical application refers to the jacks which perform bending actions on the chocks of the rolling rolls, the back-up rolls and/or the working rolls, in a four-high rolling stand for sheet or strip.
The jacks are normally arranged on both sides of the chocks and act alternately with a thrust action so as to generate positive or negative bends on the relative rolls so as to compensate the different deformations of the rolls which are caused by the rolling forces.
The working rolls, together with the back-up rolls, may be subjected, according to the state of the art, to shifting and/or crossing actions which determine a variation in the reciprocal positioning, both axial and angular, of the compression element and the surface which is subject to compression.
Document DE-A-28 04 007 shows a balancing, bending and supporting device for rolls in rolling stands consisting of a piston suitable to act on the chocks of the rolls.
The device described in DE'007 is suitable to act on chocks which are not subject to ample oscillation movements on a plane orthogonal to that on which the piston acts, such as those determined by crossing or shifting movements during the rolling passes, but which are subject only to minimum movements deriving from the play between the chocks and relative supporting elements.
The main purpose of the device described in DE'007 is to eliminate wear on the chocks provided between the piston rod and the walls wherein the rod slides when the slight displacement of the chock inclines the rod with respect to the axial direction and takes it against the chocks.
The device has a piston with a hollow rod containing inside itself, with a defined play, an axial pin equipped with ends shaped like a spherical cap.
The inner end of the axial pin cooperates with a mating spherical shaped seating provided inside the piston rod, while the outer end of the axial pin cooperates with a plane supporting plate made in the chock.
In proximity with the outer end the axial pin has an elastic ring which, cooperating with the walls of the axial hole of the piston rod, has the function of centering the axial pin with respect to the piston rod, and also of restoring the axially centered position thereof after any possible inclination of the said pin caused by the displacement of the chock.
A first disadvantage of this solution is the very limited travel of inclination which the axial pin can assume inside the hole of the piston, which makes it completely unsuitable for use in stands equipped with crossing and/or shifting movements.
Another disadvantage is that the elastic ring, because of its section, is limited in its ability to take the axial pin back on axis with the piston rod.
A further disadvantage is the lengthened and not very compact structure of the piston and the relative rod, which make it unsuitable to support high bending and compensation loads and efforts.
Moreover, the fact that the outer spherical surface of the axial pin acts and slides on a plane surface causes a rapid wear at points of the said surface in correspondence with the area of contact; this generates forces of friction which grow gradually greater and compromise the efficiency of the action of compression.
DE-A-1 527 642 also describes a hydraulic adjustment cylinder with a relative piston which exerts an action of axial compression on the chocks of the rolls in a rolling stand.
In this case it provides that the piston is made in two parts, which are subject to the action of respective flows of oil fed in two distinct zones of the cylinder.
The hydraulic feed made distinctly to the lower and upper parts of the piston causes a yielding coupling of the chock and the pressure element of the piston, which can thus move laterally inside the piston, discharging any possible lateral impacts without the relative stresses affecting the piston.
This solution, like the previous one, is suitable only for minimal lateral displacements of the chocks caused by design play, but not for the ample displacements caused by crossing and shifting movements.
Moreover, this solution does not include any elastic elements to restore the axial position of the thruster element.
Document DE-A-2261991 shows a solution which is similar to the previous one and has the same shortcomings mentioned above.
One solution to the aforementioned problems has been supplied by EP-A-489.306.
This document shows a compression unit comprising a plunger which is axially movable housed in a stationary containing seating. The plunger can oscillate at an angle inside the seating and is associated at its lower part with a floating block defining a spherical connection seating mating with the lower end, in the form of a spherical cap, of the plunger.
The lower surface of the floating block faces towards the base surface of the stationary containing seating and cooperates therewith by means of elastic contrasting means.
At the upper part, the plunger cooperates with centering means consisting of an inclined plane edge on which a segment of surface, shaped like a conical ring, of the plunger itself rests.
The angled oscillation of the plunger is achieved by making the plunger retreat on its axis, against the action of the elastic means included on the lower part, in order to release the plunger from the centering constraint of the conical edge until the lower base of the floating block is made to abut against the base surface of the stationary containing seating.
Then, the plunger is free to oscillate at an angle thanks to the sliding of the coupled spherical surfaces.
This system has a plurality of disadvantages: it has connecting surfaces of a limited extent; it requires the plunger to travel axially, firstly to release itself and then to return to the centered position; it uses a spring outside the plunger to return it to the original axial position, and it uses inclined plane centering means which achieve centering by coupling the surfaces.
To be more exact, the centering action can be achieved only when the surface on which the thruster element of the plunger is acting is distanced therefrom, that is to say, when the plunger does not exert any compression thrust on the said surface.
Moreover, the centering occurs mechanically due to contact between two conical surfaces, and therefore not due to the presence of elastic elements.
In the long term, this leads to localised wear and imperfections in the positioning.
The device described in EP'306 also has an lengthened structure; it is not compact and is not suitable to transmit high forces of compression and compensation.
It should also be stressed that it is complex to achieve and complex in functioning.
The present applicant has designed, tested and embodied this invention to resolve all these disadvantages and to provide a solution which is simpler, more rational, more functional and inexpensive.